NPWT, often referred to as topical negative pressure (TNP) or vacuum assisted closure, has been shown to be extremely useful in the treatment of many wound types including but not limited to chronic, complex acute wounds by making the healing thereof faster and more controlled. Further, NPWT has been shown to be useful in the treatment of burns, flaps, grafts and incisional wounds. It is to be understood that the term wound may have a broad interpretation and may include damage to or loss of soft tissue in a mammalian body. The apparatus used for applying NPWT generally includes a drape or sealing film or similar to create a closed environment over the wound. An aspirant conduit is brought into fluid communication with the closed environment and connected at a distal end to a vacuum source, such as an electrically driven pump or manual pump for example, to create a negative (reduced) pressure within the wound cavity compared to ambient pressure. The reduced pressure causes many beneficial therapeutic effects to the wound such as increased blood flow, faster growth of granulation tissue, and removal of exudates away from the wound, for example.
NPWT can be used to treat wounds of many shapes and sizes. The wounds may also have significant depth and therefore significant volume. Clinicians continue to require enhanced outcomes from the modern NPWT dressing. In particular, large wounds where considerable tissue loss has been experienced by the patient often require rapid growth of (tissue before closure can take place. In such cases rapid formation of granulation tissue is desirable in order to fill the defect in the tissue and promote wound contraction and finally re-epithelialization.
Preferably wounds should heal from the base up, and close in from the edges, desirably in a uniform manner. In particular it is desirable that the wound does not close over and form an occluded cavity or ‘dead space’ in the tissue, as such a cavity would be vulnerable to infection.
To prevent the formation of occluded cavities during NPWT, the wound may be packed with a filler that desirably has some resilience to resist the compressive forces created during NPWT, yet allows transmission of negative pressure and fluid flow. A purpose of the filler is to keep the edges of the wound apart so that they cannot grow over and form such cavity. When negative pressure is applied to a wound site, there is a tendency for the filler to collapse and be pushed towards the wound bed. The filler may be shaped by the clinician to fit the particular wound and placed in the wound to form intimate contact with the wound bed.
The filler may also provide fluid flow channels in order to provide a uniform reduced pressure distribution over the surface area of the wound and to promote efficient aspiration of fluid exudates away from the wound surface (generally into a remote waste receptacle associated with the aspirant conduit or into a storage area within the wound dressing itself). The presence of a wound filler may also stimulate growth of new tissue by subjecting the underlying tissue to a degree of stress. It is well known that application of stress to the cells in the wound resulting from the topography of the wound filler imparting strain on the wound surface is an important factor in stimulating cell proliferation and increasing the production of extracellular matrix. It has been shown that by increasing tissue strain and thus increasing cell stress, proliferation of cells can be increased.
Known wound fillers often consist of open-celled foam, such as reticulated foam, or gauze. Both these types of filler allow good transmission of negative pressure and allow fluid removal, yet suffer from various drawbacks. Foam fillers often suffer from the fact that tissue can grow into the foam structure. The foam may become stuck to the wound bed, making the filler difficult to remove when changing the dressing. Newly formed granulation tissue may be torn away with the foam when the filler is removed, which may cause patient pain during removal of the filler. This can be traumatic to the wound and to the patient. The clinician is often faced with having to compromise between changing a dressing early to keep tissue in-growth to a minimum and leaving the dressing in place to minimize nursing time, treatment cost and patient access. This is a particular problem with current open pore foam fillers (i.e. foam having a very open pore structure). Thus use of open pore wound fillers tends to be limited to 2 to 3 days, beyond which significant tissue in-growth and subsequent attachment is thought to occur, at least potentially resulting in damage to the tissue and pain on removal. Gauze fillers and mixed open-cell/closed-cell foam fillers (e.g. poly vinyl alcohol based foam) generally perform better with respect to in-growth, but may be inferior in their ability to induce comparable levels of observed granulation tissue. It is well known that the inclusion of a wound contact layer located between the filler and the wound surface reduces the chance of tissue growing into the foam, although again this is to the detriment of reducing the observed granulation tissue formation. Healing time may also be lengthened as a result. In many circumstances a wound contact layer is the term given to a thin sheet or membrane of material that may be positioned directly onto a wound bed. However, a wound contact layer could be any layer or member that contacts a wound bed.
A number of attempts have been described in the prior art to limit tissue in-growth into a filler. However, this has always been at the expense of limiting granulation tissue growth and thus overall clinical efficacy. U.S. Pat. No. 6,695,823, US2007/0293830, US2008/091521, US2006/046060, US2008/0317826, US2009/0105671, US2008/0300555, WO2008/141228, US2010/0160876 and WO02009/089016 describe such attempts.